


A Joint Effort

by SoloShikigami



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap (Undertale), BDSM, Breeding, Mpreg, Multi, Oviposition, Soul Sex, Threesome, Threesome - F/F/M
Language: English
Status: Completed
Published: 2021-02-11
Updated: 2021-02-11
Packaged: 2021-03-17 08:13:00
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,138
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29347179
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Alphys, Undyne, and Sans are going to work together to have a family.Please read the note at the beginning of the story!
Relationships: Alphys/Sans/Undyne (Undertale)
Comments: 1
Kudos: 28





	A Joint Effort

**Author's Note:**

> This fic is written for Yastaghar!  
> Small warning for switched up genitalia on all three of the characters, not sure if that would bother anyone and there's not really a tag for it?  
> Otherwise, enjoy!

"Sans, are you sure you're okay?" Undyne asked, fidgeting a little.

"Undyne, we've talked about this," Sans said, smiling at her and trying to exude as much reassurance as he could.

"I know, I know, it's just," Undyne shook herself, took a deep breath and let it out slowly. "I just appreciate you doing this for us."

"It's for all of us," Alphys said.

"That's right!" Sans said brightly. "Besides, my soul is strong enough to handle them!"

"Maybe mine could be too, though!" Undyne tried to argue.

"Baby girl," Alphys said, reaching over to cup Undyne's jaw. "We already learned that it's a little dangerous for you, and we know I can't carry them because it will put me at risk at work."

"Yeah! I stay home as it is, and this way, I can be a part of it too!" Sans said brightly.

"I know, I just want to be sure that you are okay," Undyne said.

"I am!" Sans leaned forward and kissed her. "I promise, and the Magnificent Sans always keeps his promises!"

"He sure does," Alphys purred, tugging at the collar he wore a little, just enough to catch his attention for a kiss.

Undyne watched them, her hand reaching over to trail up and down Alphys' thigh, recalling how lucky she was to have such lovely partners. She had a crush on Alphys since the moment she met her in Waterfall; she was tough but kind, blunt but sincere, she was also reliable, funny, and oh, how Undyne did enjoy being held in her strong arms.

At first, Undyne saw Sans as, well, not really a threat per se, even saying competition was a little harsh, it was just that Undyne felt surely with him around, Alphys would never feel the way she felt about her. Sans was adorable, a ball of energy, could hold his own in a sparring match with the Captain of the Royal Guard, friendly, and she knew Alphys was giving him private one on one training sessions.

And don’t think her anime fangirl mind didn’t take that idea train to a ton of stations.

Thankfully, it all worked out when Alphys started having Undyne and Sans hang out with each other, and not necessarily when she was around. Sans and Undyne shared a love for anime and she liked his cooking, and soon their feelings began to grow for each other. It was a little shocking to both Undyne and Alphys when Sans was the one who had the emotional breakdown, declaring that he loved and adored them both, but there wasn't any reason they should be with him due to his lack of popularity and the obviousness that they both loved each other. Alphys and Undyne had to come to terms quickly with their feelings about one another, as well as their feelings for Sans. A few discussions, a handful of dates, and some time later, the three of them found that they worked best as a unit.

Alphys was the head of their unit, claiming Sans with a collar and Undyne with a set of cuffs, both marked with her magic. Sans' collar was also marked with Undyne's magic, as she was Alphys' second. This arrangement worked for them all, as it allowed Alphys to protect her lovers, it gave Undyne confidence and helped her work through her guilt issues from her past, and it gave Sans boundaries and structure, and it helped calm his more rambunctious side.

But tonight wasn't going to be about their dynamic; they wanted to start a family, and a with everything else they did, they wanted to do it together.

"Show me your soul, Sans," Alphys murmured against his mouth when she broke their kiss.

"Yes, ma'am," Sans replied softly.

It didn't take long for Sans to summon his soul, and he held it out to Alphys. The only thing Sans was wearing was his collar, and his magic formed a pale blue body over his bones. Undyne was also nude except for her cuffs and a necklace that held the keys to Sans' collar. Alphys wore a tight-fitting, cropped tank top; her top surgery left her chest very sensitive, so she felt the most comfortable wearing it at all times.

It was a small bit of contention between her and Sans during shower time

"Good boy, thank you," Alphys said, nuzzling the side of his face as she carefully took it in her hands. "Undyne, would you take care of our boy?"

"Of course," Undyne said, settling behind Sans and pulling him against her, pressing her breasts again his backside, her legs lined next to his, and she wrapped her arms around his torso, nuzzling into his neck and grazing gentle fingertips over his magic.

Sans wriggled a little, trying to settle back against Undyne, but Alphys was gently stroking the surface of his soul and it made it hard for him to sit still.

"Easy, easy," Undyne said, leaning down to nibble along his neck. "You're going to make me too hard, too soon."

Sans sighed, tilting his head to make it easier for Undyne, but a little unable to help wriggling his ass a bit against the growing cock beginning to poke at him. 

"Brat," Undyne murmured lovingly.

Alphys smiled and continued to stroke Sans' soul, then she brought it to her mouth and licked a long line across the surface. Sans hissed, but Undyne bit him a little harder and he stilled with a small whimper.

"Mmm, excited already, Sansy?" Alphys murmured.

"Yes," Sans said, trying to keep the whine out of his voice.

"He's quite eager," Undyne said, one hand sliding down between his legs. "He even has his pussy ready for us."

"So soon? Well, then," Alphys said, moving closer to the two. "I guess you better get to work."

Undyne smiled and shifted them both down a bit more on the bed, Alphys moved in closer, bending to set Sans' soul on his chest. It reabsorbed, but the bright shape moved downwards instead of settling where it belonged, down to his pelvis, where it waited. Alphys moved to straddle both Undyne and Sans, presenting her cock in front of the skeleton's mouth. He moved to take her in, but then hesitated and glanced up, so used to waiting for permission.

"It's okay, Sansy," Alphys assured him.

Sans didn't waste a second and nearly swallowed all of Alphys down in one go. Her eyes widened and she set a hand against the wall and groaned.

"Fuck, you really are eager," Alphys said.

Sans sucked on Alphys the way he knew she liked, concentrating his magic so his tongue was a little longer and flexible, wrapping it around the thick rod. It didn't take Alphys long to groan and start thrusting her hips forward as if she were chasing the sensation that Sans was happily providing.

"Such a good boy, Sansy," Undyne said, stroking the back of his head. "Such a sexy boy, too, swallowing that cock down as if it were made for you."

"Mmm, your mouth, feels so good," Alphys breathed.

Undyne murmured encouragements, touching both Sans and Alphys, enjoying their sounds of pleasure, the small moans and heavy panting breaths. She slid her hand between Alphys' legs behind the base of her cock, feeling for the sack of eggs that were going to be nestled into Sans' soul. Her skin was warm, almost hot to the touch, she could feel the eggs - perhaps a little smaller than golf balls - and there was a twitch which indicated Alphys was ready.

"Last chance little one," Alphys said, looking down at Sans.

He hummed, pulling off her cock and looking up, his eye-lights star-shaped and bright.

"I want you, please," Sans murmured.

Alphys groaned, she loved it when either of her lovers said that while looking at her with such desire and need. 

"He is quite ready," Undyne assured her. "I've been prepping him."

"Good girl, thank you," Alphys said, leaning in to drop a kiss on Undyne's forehead before settling her bulk between Sans' legs.

Barely able to wait another moment, Alphys guided herself to Sans' entrance, giving a glance to check for any last moment hesitation, but when those bright eyes spoke the plea that Sans let out in a small whine, she grinned and eased herself into his warm, wet entrance.

Sans gasped and pressed back against Undyne.

"Are you all right?" Undyne asked.

"B-big," Sans stammered.

"Should I stop?" Alphys asked, stilling herself.

"Don't you dare," Sans breathed. "Just, slow, please."

"Of course."

Undyne licked a finger and gently rubbed small, slow circles over Sans' clit to help keep him relaxed. He tried to control his breathing, caught between wanting to back away and wanting to grab Alphys by the hips and pull her into him. He could feel that her eggs were already starting to make their way through her channels, he could feel the slight bumps as she continued to press into him.

Sans gasped when Alphys was fully inside, his body shuddering and squeezing her cock.

"Damn," Alphys grunted. "A bit early, Sansy."

"Oh, I probably didn't help much," Undyne said, blushing a little but not really sorry.

"That felt really good though," Sans said breathily. "Mmph, but please, Alphys."

"Easy, easy," Alphys purred, stroking Sans' face and leaning over him to kiss Undyne. "Ready to impregnate him?"

"I've been ready all day," Undyne murmured.

Alphys smiled, kissing Undyne again as she carefully moved. She went slow, making sure she wasn't going to accidentally hurt the little skeleton beneath her, but she also knew he was tough as nails and his squirming began to indicate that he wanted more. She worked up to a steady, medium pace, and she felt a mix of pleasure and discomfort as the eggs reached the tip of her cock, ready to be passed to Sans.

"They're ready, are you?" she asked Sans.

"Yeah," Sans said.

"Such a good boy, mmph," Alphys grunted a little. "Here comes the first."

She sank deep into Sans, wrapping one arm around him, holding him close, the other grabbing Undyne's hand, and she let out a small cry as the first egg passed out of her. Sans gasped as he felt the egg enter his soul; a little uncomfortable, but for the most part it was an odd weight and nothing else.

"Are you all right?" Undyne asked.

"Yeah," Alphys said with a relieved sigh. "The rest should be easier. How are you, darling?"

"Good," Sans said with a nod.

The three shared a smile, happy that this process was off to a good start. Alphys slowly started thrusting again, though she reached the moderate pace a little faster than before.

"Here comes the next," she warned Sans.

She thrust in deep, passed the egg, this time it was easier on them both, and they continued. Undyne touched them, murmured encouragements, and watched with fascination as Sans's soul and belly slowly began to swell with each egg that Alphys gave him. It was beautiful to watch this merging of the two monsters she loved, admiration for Sans' endurance and the beauty of Alphys' creation.

It didn't take much longer when they got going, and Sans was filled with about twenty of Alphys' eggs. Alphys breathed a sigh of relief, and Sans moaned; he had cum again in the middle of the process, and now he was filled with the weight of the eggs.

"How are you doing?" Undyne asked.

"Good, feel a little heavy and weird, but, good," Sans sighed, then he looked up at Undyne with a grin. "Now we just need you to fertilize them."

Undyne blushed a little, Sans rolled over and kissed her, rubbing his body against hers, reaching one hand down to stroke her cock, which had been getting harder while she watched Alphys and Sans. It never failed, she loved watching them together, and the fact that they didn't mind fulfilling some of the fantasies that she had previously only felt brave enough to put into fanfiction helped a lot, too.

"Are you still okay with our usual position?" Undyne asked.

"I think so," Sans said as Undyne moved so Sans could lie down.

"Let's get you some pillows," Alphys said, grabbing not only the extra pillows but the blankets as well, starting to form a kind of nest around Sans.

"Alphys, that's a bit much!" Sans giggled. "And it's okay, they're safe, I promise!"

"Hey, nesting is natural," Alphys said. "And it's my job to protect you, make you comfortable."

"Alphys, I promise, it's all going to be all right," Sans said gently, getting up onto his knees and leaning into the large lizard monster. "I know what I'm doing, I can protect them."

"And I can protect you," Alphys said, kissing him. "And you as well, nerd."

She pulled Undyne in for a deep, passionate kiss, one hand raking down the fish monster's body possessively. Alphys then gently massaged her little breasts, playing with her hardened nipples, drawing soft moans from her mouth.

"Now, you're going to fuck our little boy properly, aren't you?" Alphys murmured.

"Absolutely," Undyne said breathily.

"Fill him up good, make sure he knows he belongs to you."

Undyne nodded.

"Ah, ah, I like to hear that pretty voice," Alphys said, squeezing the back of Undyne's neck slightly.

"Yes, Captain," she murmured.

"Good girl," Alphys kissed her again. "You're in control."

Undyne nodded with a smile. Alphys took any and every opportunity to help keep Undyne's confidence up, especially in the bedroom. She was proud of her progress, and while they weren't going full out in their usual roles, again, Alphys was being opportunistic.

Sans smiled, and when Undyne looked his way, she saw that he had made himself comfortable lying on his front, his belly supported and cradled, and he was wriggling his behind.

"I'm ready for you," Sans said teasingly, his eyes sparkling.

Undyne felt a pang of desire and moved in to claim her prize. She took her time, caressing the rounded ass before her, trailing her fingers between his legs to touch lightly at his entrance, biting her lip at the wetness there, evidence of his joining with Alphys as well as his own desire.

"Please, Undyne, make love to me," Sans begged, looking over his shoulder.

"Easy," Undyne said with a smile, steadying him as she lined herself up, then thrust into him with one stroke.

Sans cried out in surprise and shock; Undyne wasn't as thick as Alphys, but she was big enough, and she usually went slowly. She gripped onto Sans' hips and drove into him hard and fast. Alphys was also a little taken aback by Undyne's sudden force, but she had to admit it turned her on immensely. She looked over at Sans, who was already moaning, and with the supportive setup around him, she knew he was fine.

"Damn, babe," Alphys said, moving behind her, trailing her hands and claws lightly over her body. "You're so hot when you take charge like that."

"It was tough waiting for you two to finish, but it was so worth it," Undyne said.

"Watching you fucking our little boy, knowing that you're going to fill him and fertilize my eggs, so hot."

Alphys raked her claws down Undyne's back, leaving pale streaks behind them. Undyne arched and thrust into Sans deep, making him cry out along with her. Alphys growled and moved behind Undyne.

"Any objections to claiming what's mine?" she asked, biting Undyne's neck lightly.

"Never," Undyne gasped. "Do it."

Alphys smirked, waiting a moment, getting herself set, then thrust into Undyne in time with her thrust into Sans. Sans whimpered, feeling Alphys' power behind Undyne and he held onto the pillows tight. Alphys didn't have to move much, giving Undyne the control of their pace, thrusting into Sans and back against Alphys, between the sensation and Alphys touching her body, she knew she wasn't going to last long.

Sans certainly didn't, as he came again not long after Alphys joined in.

"I'm cumming," Undyne warned.

"Do it, pump him full," Alphys growled, gripping Undyne's shoulders and taking over a little.

Undyne gasped, thrusting into Sans deep once, twice, the third she stilled and Alphys held her there, pounding into her as she came inside of Sans. Sans' walls fluttered around her, helping to milk her, soft whimpers of pleasure coming from his mouth as his whole body trembled. Undyne didn't move, Alphys wrapped her arms around her, and all three were still for a moment.

"Did you want to-?" Undyne asked.

"No, I'm good," Alphys said, kissing the bite she left on Undyne. "So good."

They checked in with Sans, who was a little out of it. They lay together and cuddled the small monster, supporting him on either side, the three sharing kisses and soft touches, especially over the belly that formed where Sans’ soul held the eggs, and they watched as they absorbed Undyne's magic, their healthy glow indicating that they were fertilized and the cycle of life was beginning.

"So beautiful," Undyne murmured.

"You both certainly are," Alphys said with a smile.

"I think she meant the eggs," Sans said.

"Of course, but that's no surprise," Alphys said.

"Flatterer," Undyne giggled.

"You bet."

Alphys leaned in to kiss both of her lovers again.

"Are you ready for a bath, Sans?" Undyne asked.

"Not yet," Sans said with a small sigh, surprising his partners. Sans was usually the first to insist on getting them into the bathroom for cleaning. "Just hold me a little longer?"

"Of course," Alphys said.

"Always," Undyne agreed as they both snuggled in closer.

"Are you okay, sweetie?" Alphys asked, noticing Sans looking a little pensive.

"Yes, I am more than okay," Sans smiled at them. "Just very tired, you both worked me over really good. Also, well, them."

He wrapped his arms around his own belly.

"Are you-?" Undyne asked.

"I'm happy, no regrets," Sans assured her. "There really is no turning back. I love you both so, so much, I just hope I don't let you down."

"Impossible, you never could," Undyne assured him.

"The Magnificent Sans can do anything," Alphys added. "And hey, you're not doing this alone, you know?"

"I know, and I am so grateful," Sans turned and kissed each of them. "I love you both so much."

"I love you, too," Undyne said.

"And I love all of you the most!" Alphys said loudly, getting both arms around Sans and Undyne and pulling them in for a tight (but not too tight) hug.

The three of them shared another loving look, then looked down at the life they came together to create, and knew they were about to embark on an incredible journey.

**Author's Note:**

> Like what you read? Please check out my social media for more fun!  
> [ My Tumblr](https://soloshikigami.tumblr.com/)  
> [ My Twitter](https://twitter.com/SoloShikigami)


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